Switching devices, in particular low-voltage switching devices, can be used to switch the current paths between an electrical supply device and loads, and therefore to switch their operating currents. Thus, the switching device opens and closes current paths, allowing the connected loads to be safely connected and disconnected.
An electrical low-voltage switching device, such as a contactor, a circuit breaker or a compact starter, has one or more so-called main contacts, which can be controlled by one or else more control magnets, in order to switch the current paths. In principle, in this case, the main contacts include a moving contact link and fixed contact pieces, to which the loads and the supply device are connected. In order to close and open the main contacts, an appropriate connection or disconnection signal is passed to the control magnets, in response to which their armatures act on the moving contact links such that the latter carry out a relative movement with respect to the fixed contact pieces, and either close or open the current paths to be switched.
Appropriately designed contact surfaces are provided in order to improve the contact between the contact pieces and the contact links at points at which the two meet one another. These contact surfaces are composed of materials such as silver alloys, which are applied at these points both to the contact link and to the contact pieces, and have a specific thickness.
The materials of the contact surfaces are subject to wear during every switching process. Factors which can influence this wear are:                increasing contact erosion or contact wear as the number of connection and disconnection processes increases,        increasing deformation,        increasing contact corrosion caused by arcing, or        environmental influences, such as vapors or suspended particles, etc.        
This results in the operating currents no longer being safely switched, which can lead to current interruptions, contact heating or to contact welding.
For example, particularly as the contact erosion increases, the thickness of the materials applied to the contact surfaces will decrease. The switching movement between the contact surfaces of the contact link and the contact pieces therefore becomes longer, thus in the end reducing the contact force on closing. As the number of switching processes increases, this results in the contacts no longer closing correctly. The resultant current interruptions or else the increased connection bouncing can then lead to contact heating and thus to increasing melting of the contact material, which can in turn then lead to welding of the contact surfaces of the main contacts.
If a main contact of the switching device has become worn or even welded, the switching device can no longer safely disconnect the load. In particular in the case of a welded contact, at least the current path with the welded main contact will still continue to carry current and will still be live, despite the disconnection signal, so that the load is not completely isolated from the supply device. Since, in consequence, the load remains in a non-safe state, the switching device represents a potential fault source.
The protective function can thus be blocked, for example, in the case of compact starters according to IEC 60 947-6-2, in which an additional protection mechanism acts on the same main contacts as the control magnet during normal switching.
Fault sources such as these must therefore be avoided for safe operation of switching devices and thus for protection of the load and of the electrical installation.
European Laid-Open Specification EP 1 002 325 A1 discloses a relatively complex method for identification of the remaining electrical life of contacts, in which contact welding during disconnection of the switching device is identified by existing or additional means. The risk resulting from major electrical faults for loads and electrical installations is thus overcome by emitting a message and/or by ceasing switching operation, in particular after short-circuit switching operations.
European Laid-Open Specification EP 0 832 496 A1 discloses a method in which contact welding in the switching device is detected by monitoring the switching device drive. A series-connected second switching device is operated in order to interrupt the circuit when the switching device drive does not reach its normal disconnected position during the disconnection process.